/Engine/Source/Runtime/SlateCore/Public/Widgets/Images/SImage.h
an attribute reference of DesiredSizeOverride
[TSlateAttributeRef](API\Runtime\SlateCore\Widgets\SWidget\TSlateAttributeRef)< [TOptional](API\Runtime\Core\IO\TOptional)< FVector2D > > GetDesiredSizeOverrideAttribute() const